I'm getting the same error. When accessing the Community Server and then selecting Documents this error pops up in the top-right corner:

This happened after attempting to apt-get upgrade packages on my server. When asked if I wanted to update OnlyOffice I chose the option to see the differences between the releases. The server froze and I had to restart it. When I ran the upgrade command again it said that there were errors. After some digging I found that I had to edit the nginx sites-enabled file to get nginx to run. Then I ran apt-get upgrade and did not receive any errors after OnlyOffice did it's thing. The Community Server seems to be running okay, but I cannot access any Documents at all.

I don't know if these lines have anything to do with the error, but these are the two lines I had to alter for nginx to run again. The first line was in the onlyoffice file. I removed the part that said text/html because it was a "duplicate mime type":